Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
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As Iava cools and consolidates, it characteristically deveIops a vertica1 cohmmar<br />
jointing, the columns being at right angles to the cooling surface. This verticaI<br />
jointing is so we11 developed that dissection <strong>of</strong> a succession <strong>of</strong> Iava flows <strong>of</strong>ten<br />
produces a step-like pr<strong>of</strong>ile, the treads being controlled by the individual horizontal<br />
flows, and the risers by the vertical joints in them. Vertical cliffs, each the height<br />
<strong>of</strong> a flow’s thickness, or escarpments hundreds <strong>of</strong> feet high, are characteristic <strong>of</strong><br />
areas <strong>of</strong> dissected lava.<br />
Figure 4. Diagram showing the main areas <strong>of</strong> flat-lying or gently dipping lava flows.<br />
Figure 4 shows the area1 distribution <strong>of</strong> flat-lying or gently dipping lava flows.<br />
They occur 1argeIy within the southern and central interior <strong>of</strong> the Province, and<br />
extend over considerable areas between Penticton in the south and Houston in the<br />
north. A very large area west <strong>of</strong> the Fraser River in the drainage basin <strong>of</strong> the<br />
Chilcotin, West Road, and Nechako Rivers is underlain by flat-lying lavas (see<br />
Plate XXVI.4). Comparable rocks underlie areas <strong>of</strong> considerable size centring<br />
around Mount Edziza and Meszah Peak (see Plate XVIIB) and in the Kawdy<br />
Plateau west <strong>of</strong> Tuya Lake. Gently dipping lavas underlie a somewhat smaller<br />
area in the northwest part <strong>of</strong> Graham IsIand (see Plate 11~).<br />
(4) The tilting <strong>of</strong> sedimentary rocks allows the differences in resistance<br />
between beds to control the topography. Streams erode the s<strong>of</strong>ter beds faster, and<br />
consequently peaks and ridges are underlain by more resistant beds than are the<br />
valleys and depressions. Folding and faulting <strong>of</strong> sedimentary rocks develop<br />
structures in which tilted beds may extend for many miles, and erosion <strong>of</strong> this<br />
sort <strong>of</strong> terrain results in topography <strong>of</strong> a very strongly linear character. The<br />
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